The present invention relates to a method for producing shaped bodies.
A method of this kind is disclosed in BE-A-653,349 and SE-B-304,711 (both based on FR priority application No. 955,561 of Nov. 29, 1963). In this known method, an unhardened mixture comprising hydraulic cement and aggregate material (sand and gravel) with surplus water is compressed in an extruder of constant cross-sectional shape by means of a reciprocating piston, and in the terminal part of said extruder, the walls of which are suitably perforated, part of the water is removed by applying a vacuum to the outside of said walls, all this taking place while the material is moving slowly through the extruder.
Obviously, the pressure differential that can be produced by said vacuum arrangement is at the highest of the order of one bar. In addition to this, the reciprocating piston does, admittedly, exert a certain force, thus causing a corresponding increase in the pressure differential effecting the de-watering, but if sufficiently increased, this force will simply push the material out of the extruder, as no counter-force is provided to prevent this.
This means, of course, that the total pressure differential across the perforated walls will at the most be of the order of a few bar. This in turn means that the ability of this previously known method to remove liquid from the spaces between the particles of the material is limited, and in many cases the quantity of the remaining liquid is sufficient to prevent the shaped bodies produced from attaining more structural strength than just needed to keep their shape against the force of gravity, so that they, unless extreme care is taken, cannot be handled without deforming, collapsing or falling apart.
The above problem is, of course, less serious in the case of shaped bodies of clay, as such bodies can be allowed to or be made to harden respectively be well-known methods before being moved, but the method referred to above is obviously insufficient, if the shaped bodies are to have a reasonable strength immediately upon having been produced by carrying out the method.
It is the object of the present invention to provide a method of the kind referred to initially, with which it is possible to produce shaped bodies having a considerable mechanical strength, so that they can be handled or manipulated mechanically immediately upon completion of the final step of the method without any risk of deforming, collapsing or falling apart.
By proceeding in this manner, the high pressure differential, produced by applying a high positive pressure to the inside of the perforated walls in the mould, will cause so much of the liquid between the particles to be expelled and the particles to come into such mutual engagement, that a shaped body having a considerable mechanical strength is produced, and as the slurry has already been homogenized, the shaped body will have a uniform structure throughout its volume.
If the squeezing-out of the liquid occurs at the same time over the whole surface of the mould, there is a risk that dewatered and un-dewatered material moves about uncontrollably in the moulding space with the result that the end product does not become fully homogeneous. This disadvantage may be avoided by proceeding as set forth by the use of a mold, in which the perforations are distributed and adapted in such a manner so that the liquid will be expressed first from the parts of the mold situated most distant from the slurry inlet, then from parts of the mold less distant from said inlet, then from parts still closer to the inlet and so forth, until the complete molding space is occupied by closely packed and consolidated particulate material forming a compact body with very low porosity.
When proceeding in this manner, the final part of the pressing process, when no further water can be squeezed out, can be characterized as powder pressing.
Thus, the process as such commences in the form of high-pressure slurry pumping in one end of the mould and terminates as a powder-pressing process steadily progressing from the other end of the mould. It will be understood that in this case, the low-viscosity suspension will have no difficulty in flowing out into all nooks and crannies of the mould, and any air having been trapped during the filling-up of the mould will leave the mould cavity through its perforations together with the surplus liquid. The finished press-moulded object will constitute an accurate replica of the internal surfaces of the mould, and since the composite material already has solidified in the mould in the same moment as all surplus water has been squeezed out and mutual contact between the solid-matter particles has been achieved, it is now possible to remove the moulded object from the mould immediatelyxe2x80x94just as with any other powder-pressing methodxe2x80x94since this object is now fully rigid and self-supporting and requires no more than being allowed to harden completely by hydration in a suitable manner.
Similar results with regard to making the dewatering and consolidation process progress steadily from one end or side of the mould to the other may be achieved by A) using a mold in which the liquid-permeability of the perforations diminishes steadily from the end of the mold most distant from the inlet towards the latter so as to make the removal of the liquid occur at the highest rate at said most distant end and at a steadily diminishing rate when approaching the inlet or B) use of a mold in which the perforations may be closed and opened from the outside, the removal of the liquid being carried out by opening the perforations in a sequence beginning at the point in the mold most distant from the inlet and ending at the latter.
The perforations or holes in the walls of the moulds should, of course, be extremely fine, so that the water, but not the solid-matter particles may escape from the mould, but since water molecules are extremely small (approximately 20 xc3x85), this should not be a problem.
The end product made by proceeding according to one of the embodiments of the method according to the invention is characterized by being exceptionally dense and with an absolute minimum of porosity and being highly homogeneous, and by, in the fully-hardened condition, to possess valuable physical properties comprising an optimum combination of strength and toughness.
Since, as described above, the mixing process is carried out with an arbitrary surplus amount of liquid, and the concentration of the material subsequently during the casting or moulding process is increased without xe2x80x9cde-mixingxe2x80x9d taking place, until no more liquid can be squeezed out from the confined material, it is possible in this case to achieve a considerably higher concentration of fibres in the end product than by using any other known moulding or casting principle, still with the fibres lying fully dispersed and well distributed and oriented throughout the product.
During the terminal part of the pressing process, during which the solid particles are closely wedged and pressed together, so that the material solidifies, the particles are also pressed firmly against all fibre surfacesxe2x80x94in certain cases even into the surfaces of the fibresxe2x80x94resulting in optimum bond between the fibre and the matrix material and hence optimum fibre effect in the end product.
In this process, fibres and matrix material xe2x80x9cgrow togetherxe2x80x9d in a manner not being known from other casting or moulding processes, and after having fully hardened, the end product possesses unique physical properties.
With uniaxial tension loading, which is the most problematic form of loading to such brittle-matrix materials (because it is difficult for the fibres to take over the whole tensional load when the matrix is over-strained), it is possible with a correctly reinforced BMC (Brittle-Matrix-Composite) material produced according to the present invention to achieve a stress-strain curve more reminiscent of the stress-strain curve for a metal or for a plastic material than for an ordinary brittle matrix material normally exhibiting an ultimate elongation at rupture of only approximately 0.01-0.02 percent (0.1-0.2 mm per m).
After hardening, a correctly made BMC material produced according to the present invention will have a tensile stress-strain curve exhibiting so-called strain hardening, in which the tensile stress continues to increasexe2x80x94without any formation of visible or harmful cracksxe2x80x94even right up to a strain of 1-2% or more. Thus, the strainability (elasticity or flexibility if so preferred) of the matrix material has, by extreme utilization of the admixed fibres, been increased by a factor of 100 or morexe2x80x94and this without causing any damage to the composite material.
The mechanism behind the dramatically increased strainability of the composite material is that the internal rupturing of the matrix material between the fibres due to tensile straining occurs in a different manner than in similar non-reinforced material, as, on a microscopic level, an evenly distributed pattern of extremely fine and short microscopic cracks are formed, increasing in number with increased straining of the material; these microscopic cracks are, however, so small that they may be stopped or blocked by the surrounding fibres, and for this reason they cause no dramatic damage to the material as such.
This is in itself extremely valuable and applies in general to the high-quality BMC materials mentioned above as produced by the methods according to the invention. Further, experience has shown that for so-called FRC material produced with a normal Portland-cement matrix, the network of micro-cracks formed in the manner referred to above (with possible crack lengths of approximately 0.5-1 mm or less, width typically 10-50 xcexcm) after being formed shows a marked tendency to self-healing, so that the material in the presence of moisture will again be dense, and so that the material when again being tension loaded achieves its original rigidity and strength and may be subjected to increased stresses in the same manner as during the first loading, also here exhibiting a smooth stress-strain curve and a convincing strain hardening with steadily increasing tensile stresses up to an ultimate straining capacity of 1-2% or more before the stresses begin to decrease.
The present invention also relates to an apparatus for carrying out the method of the invention.
Finally, the invention relates to a product comprising a non-flowable body of consolidated, closely-packed particles of solid materials produced by the method and/or apparatus of the invention.
Advantageous embodiments of the method and the apparatus, the effects of whichxe2x80x94beyond what is self-evidentxe2x80x94are explained in the following detailed part of the present description.